This invention relates to polyester compositions having nucleating agents to promote crystallization and to articles made by thermoforming the nucleated crystalline polyesters.
Polyesters are widely used for articles for which non-toxicity, good heat stability, resistance to deformation and ease of manufacture are important. Polyethylene terephthalate ("PET") has, for example, become the material of choice for dual ovenable (ability to be used in conventional ovens as well as microwave ovens) food packages and trays. Food trays and similar articles are generally made from polyester by thermoforming. In thermoforming, polyester sheet is preheated to its deformation temperature and made to conform to the contours of a mold by vacuum assist (vacuuming it into the mold), air pressure assist (blowing it into the mold), and matched mold assist (forcing it into the mold). In another process extruded melted sheet is cooled to the proper forming temperature before molding by the means described above. It is essential that the polyester, after forming, be partially crystalline to maintain good dimensional stability, stiffness and other desirable properties. Crystallinity of about 15 to 32 percent after forming is normally adequate. It is a happy coincidence that polyesters crystallize upon heating, so that the thermoforming step plus a period of heat treatment (holding the article at the molding temperature for a time) results in the desired crystallinity. Of course, the polyester must be properly selected, and have added to it a suitable crystallization nucleation agent, i.e. an agent (usually particles) which initiate and provide a starting point on which crystallites can form. It is also necessary to provide some improvement in impact strength (to prevent or at least reduce shattering). This can be done by adding a small amount of a polyolefin.
Thermoforming of thin walled articles from unmodified polyester films and sheeting with heat treatment in a heated mold can achieve adequate crystallinity (i.e. about 25 percent) but not satisfactory impact strength nor acceptable cycle times. The required heat treatment is also too long (i.e. 30 to 60 seconds depending upon the temperature) to attain the desired crystallinity and the article is often difficult to remove from the heated mold. These are not new problems, and the industry has come a long way in overcoming the limitations. It is known to add polyolefins and inorganic nucleating agents, or to add only polyolefins, such as polyethylene or polypropylene, to get good results. There is, however, a never ending quest for yet better processing and product properties; to make things faster and faster, and in different ways. So, there is continuing need to decrease crystallization time and increase efficiency, and to be able to achieve fast crystallization on heating from a solid sheet or in cooling from a newly extruded melted sheet. We have discovered a very good way to do that.